Conventionally, as a system for desalinating seawater, there has been known a seawater desalination system in which seawater passes through a reverse-osmosis membrane-separation apparatus to remove salinity from the seawater. In the seawater desalination system, the intake seawater is processed to have certain water qualities by a pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane-separation apparatus under pressure by a high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane-separation apparatus passes through a reverse-osmosis membrane against the osmotic pressure and is desalinated, and fresh water (permeate or desalted water) is taken out from the reverse-osmosis membrane-separation apparatus. The remaining seawater is discharged in a concentrated state of a high salt content as a concentrated seawater (brine) from the reverse-osmosis membrane-separation apparatus. The largest operational cost in the seawater desalination system is electric power expenses, and it depends heavily on energy for pressurizing the pretreated seawater up to such a pressure to overcome the osmotic pressure, i.e. up to the reverse-osmosis pressure. That is, the operational cost of the seawater desalination system is greatly affected by pressurizing energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric power expenses as the highest cost in the seawater desalination system are consumed to operate the high-pressure pump for pressurizing the seawater. Then, pressure energy possessed by the high-pressure concentrated seawater with the high salt content which has been discharged from the reverse-osmosis membrane-separation apparatus is utilized for pressurizing part of the seawater to reduce electric power. Therefore, as a means for utilizing the pressure energy of the concentrated seawater discharged from the reverse-osmosis membrane-separation apparatus to pressurize part of the seawater, there is an energy recovery device having an energy recovery chamber in which an interior of a cylinder is separated into two spaces by a piston arranged to be movable in the cylinder, a concentrated seawater port is provided in one of the two separated spaces to introduce and discharge the concentrated seawater, and a seawater port is provided in the other of the two separated spaces to introduce and discharge the seawater.
FIG. 19 is a schematic view showing a configuration example of a conventional seawater desalination system.
As shown in FIG. 19, seawater pumped into the seawater desalination system by an intake pump (not shown) is processed to have certain water qualities by a pretreatment system for removing suspended matter or the like, and then the pretreated seawater is supplied via a feed pump 2 into a high-pressure pump 3 to which a motor M is directly connected. The seawater which has been pressurized by the high-pressure pump 3 is supplied to a reverse-osmosis membrane-separation apparatus 4 having a reverse-osmosis membrane (RO membrane). The reverse-osmosis membrane-separation apparatus 4 separates the seawater concentrated seawater with a high salt content and fresh water with a low salt content, thus obtaining the fresh water from the seawater. At this time, the remaining high-pressure concentrated seawater from which the fresh water has been separated is discharged from the reverse-osmosis membrane-separation apparatus 4, and is then supplied into a pressure convention chamber 6 from a concentrated seawater port 7 via a switching valve 5.
On the other hand, the seawater which is delivered from the feed pump 2 to the high-pressure pump 3 is partly branched and taken out, and the seawater which has been taken out is supplied into the pressure exchange chamber 6 from a seawater port 9 through a directional control valve 8. The directional control valve 8 comprises a check valve unit which has a check valve 10 for allowing the seawater pressurized in the chamber to discharge only to the outside, and a check valve 11 for allowing the seawater to flow only in the direction for supplying the seawater to the chamber.
The pressure exchange chamber 6 has a piston 12 therein, and the piston 12 is arranged to be movable in the pressure exchange chamber 6 while separating an interior of the pressure exchange chamber into two volume chambers. The switching valve 5, the directional control valve 8 and the pressure exchange chamber 6 constitute an energy recovery device 1. The energy recovery device 1 is shown by a flame enclosed by dashed-dotted lines.
When the concentrated seawater in the pressure exchange chamber 6 is discharged to the outside of the chamber by the switching valve 5 and is depressurized down to atmospheric pressure, the seawater supplied into the chamber from the seawater port 9 pushes the piston 12. When the piston moves from the seawater port side to the concentrated seawater port side, the seawater is charged in an amount corresponding to the movement of the piston into the pressure exchange chamber. Then, when the switching valve 5 and the directional control valve 8 are switched to supply the high-pressure concentrated seawater into the pressure exchange chamber 6, the pressure of the high-pressure concentrated seawater supplied to the pressure exchange chamber 6 pushes the piston 12, thus pressurizing the seawater. The piston moves from the concentrated seawater port side to the seawater port side, and the seawater is discharged in an amount corresponding to the movement of the piston from the pressure exchange chamber 6.
The seawater discharged from the pressure exchange chamber 6 is supplied to the booster pump 13 through the directional control valve 8. The seawater is further pressurized by the booster pump 13 so that the seawater has the same pressure level as the discharge line of the high-pressure pump 3, and the pressurized seawater merges via a valve 19 into the discharge line of the high-pressure pump 3 and is then supplied to the reverse-osmosis membrane-separation apparatus 4. The valve 19 in the discharge line of the booster pump 13 comprises a check valve for allowing the seawater to flow only in the direction from the booster pump 13 to the reverse-osmosis membrane-separation apparatus side, and is provided to prevent the seawater from flowing backward to the booster pump 13.
Here, in the energy recovery device 1 which comprises the pressure exchange chamber 6 having the piston 12 as shown in FIG. 19, the piston can be reciprocated in the predetermined movement range. Therefore, by moving the piston in the pressure exchange chamber 6 by the same reciprocating distance, the supply amount of the seawater and the discharge amount of the seawater can be the same and constant at all times.
On the other hand, there is an apparatus comprising a pressure exchange chamber having no piston in which movement of seawater or concentrated seawater in the chamber is repeated to recover energy from the high-pressure concentrated seawater by opening and closing operation of valves provided before and after the chamber while utilizing the difference between the pressure of the seawater and the pressure of the high-pressure concentrated seawater processed by the reverse-osmosis membrane (RO membrane).
FIG. 20 is a schematic view showing a seawater desalination system which incorporates an energy recovery device using such pressure exchange chamber 6 having no piston. A process for pressurizing seawater in the pressure exchange chamber 6 directly by high-pressure concentrated seawater processed by the reverse-osmosis membrane (RO membrane) to deliver the pressurized seawater to the booster pump 13, and a process for purging the high-pressure concentrated seawater filled in the pressure exchange chamber 6 to the outside of atmospheric pressure from one end side of the pressure exchange chamber 6 and supplying the seawater into the pressure exchange chamber 6 from the other end side of the pressure exchange chamber 6 are repeated, thereby recovering the energy of the high-pressure concentrated seawater processed by the reverse-osmosis membrane (RO membrane) by the seawater.
In such energy recovery device 1 having no piston, the control of supply and discharge of the seawater to the pressure exchange chamber 6 is performed by measuring a first flow rate of the seawater discharged by the high-pressure concentrated seawater supplied to the pressure exchange chamber 6 and a second flow rate of the concentrated seawater purged to the outside of the pressure exchange chamber 6 when the seawater is supplied to the pressure exchange chamber 6, and by balancing both the flow rates or both integrated flow rates. Therefore, as shown in FIG. 20, it is necessary to provide two flowmeters comprising a first flowmeter F1 and a second flowmeter F2 for measuring the respective flow rates in the energy recovery device 1. The first flowmeter F1 and the second flowmeter F2 are connected to a sensor controller 17, and the sensor controller 17 is connected to a device controller 18. Other structural elements of the seawater desalination system shown in FIG. 20 are the same as those of the seawater desalination system shown in FIG. 19.
Next, the reason for necessitating the two flowmeters will be described. Specifically, even if the intake amount of the seawater of the pressure exchange chamber 6 becomes small, when the discharge amount of the seawater is still the same amount before the intake amount becomes small, the portion with a high salt content caused by the high-pressure concentrated seawater is discharged from the energy recovery device subsequent to the discharge of the seawater because there is no partition by the piston. Further, conversely, even if the intake amount of the seawater becomes large, when the seawater is discharged still in the same amount as before the intake amount becomes large, extra seawater is drawn into the device, and thus the incremental amount cannot be discharged from the energy recovery device.
When the former state occurs, since the concentrated seawater is supplied to the pressure exchange chamber 6 in an amount larger than the intake amount of the seawater, the salt content of the seawater supplied to the reverse-osmosis membrane becomes high. Therefore, because of the property of the reverse-osmosis membrane, when the seawater with a high salt content is supplied, the production amount of fresh water decreases. When the latter state occurs, the seawater which has been pretreated is consumed wastefully, and thus the cost of pretreatment increases relative to the production amount of fresh water.
Thus, it is important for the energy recovery device having no piston to monitor the flow rate of intake of the seawater and the flow rate of discharge of the seawater and to make both the flow rates the same and constant in maintaining the performance of the device.
Since the fluid discharged from the pressure exchange chamber 6 has a high-pressure which is about 60 atmospheric pressure, the first flowmeter F1 needs to be high-pressure specification. Further, in order to control the supply and discharge of the fluid, it is preferable for the first flowmeter F1 to use a flowmeter having a small measurement error with respect to the second flowmeter F2 separately provided, i.e., a flowmeter having high measurement accuracy.
Although there are various types of flowmeters, presently, the flowmeter having the highest measurement accuracy is an electromagnetic flowmeter, and thus the electromagnetic flowmeter having high-pressure specification is used.
However, the electromagnetic flowmeter is expensive relative to other types of flowmeters. Further, in principle, a detecting portion of the electromagnetic flowmeter is required to be brought into direct contact with the fluid to be measured, and thus it is essential for a liquid-contacting part including the detecting portion of the flowmeter to be made of a material having corrosion resistance against the seawater as a corrosive fluid. Furthermore, the flowmeter is required to have high-pressure specification, and hence needs to have a large thickness for ensuring pressure resistance of the flow passage. In short, the flowmeter is special and expensive, large in size, and heavy in weight.
On the other hand, when the seawater is supplied to the pressure exchange chamber and the concentrated seawater in the pressure exchange chamber is purged to the outside, the pressure of the discharged concentrated seawater is close to atmospheric pressure, and hence low pressure. Accordingly, the second flowmeter F2 generally does not have high-pressure specification. Specifically, since the first flowmeter and the second flowmeter often have different specifications, the first flowmeter and the second flowmeter require calibration and correction in consideration of their characteristics.
Further, since the flowmeter generally requires to make the measurement conditions constant, it is necessary to provide an entrance interval of flow before and after the detecting portion of the flowmeter. When an inner diameter of a pipe before and after the attachment portion of the flowmeter is D, the length of the entrance interval is in the range of 5×D to 10×D. Therefore, it is necessary to provide the straight pipe which is large in length before and after the flowmeter.
In this manner, turbulence of flow generated in the detecting portion of the flowmeter by the effect of expansion, shrinkage or bending of the pipe at the upstream side or the downstream side of the flowmeter can be reduced to reduce the measurement error. However, restriction on piping design arises, and layout in which the straight pipe portion is provided needs to be designed. Since the long straight pipe portion is usually provided outside the energy recovery device, installation design of the flowmeter, wiring design, signal communication, and calibration are required to be performed according to the plant in which the energy recovery device is provided. In particular, a normal seawater desalination system having a plurality of pressure exchange chambers is greatly affected as follows:
In the case of using the pressure exchange chamber, as the amount of produced fresh water increases, the amount to be processed in the pressure exchange chamber increases. However, even if the volume of a single pressure exchange chamber is enlarged or the processing cycle is shortened, such countermeasures have their own limits. Therefore, in many cases, a method in which the pressure exchange chambers are arranged in parallel in a multistage manner is selected.
FIG. 21 is a schematic view showing an arrangement configuration of flowmeters in the case where a plurality of pressure exchange chambers having no piston are provided in parallel. As shown in FIG. 21, n pressure exchange chambers 6 are provided in parallel, seawater discharged from the n pressure exchange chambers 6 and concentrated seawater discharged from the n pressure exchange chambers 6 are collected respectively in respective collecting pipes, and then the flow rate of the seawater and the flow rate of the concentrated seawater are measured respectively by the first flowmeter F1 and the second flowmeter F2. In this case, the first flowmeter F1 and the second flowmeter F2 are provided singly. However, if the number of chambers are large and the processing flow rate is large, the diameter of the collecting pipe to which the flowmeter is attached becomes large, and thus it is necessary to enlarge the size of the flowmeter. Further, the pipe serving as the entrance interval before and after the flowmeter becomes large in length. Further, in particular, the first flowmeter F1 for measuring the flow rate of the seawater at the high-pressure side becomes large in size and heavy in weight from the necessity of pressure resistance. Therefore, the first flowmeter F1 becomes special and expensive. Further, the replacement of the flowmeter at the time of failure needs extensive work, and thus the suspension of the entire system for a long period of time may be caused. In order to deal with this situation, as shown in FIG. 21, a backup line F1′ of the first flowmeter and a backup line F2′ of the second flowmeter should be prepared. However, such measures have difficulty in securing space or the like.